The present invention relates generally to the field of intravascular leads and catheters. More specifically, the present invention relates to steerable stylet for use in positioning such leads and catheters.
Stylets and guidewires are used to control the manner in which intravascular leads and catheters are introduced into the veins or arteries of the body. Although both kinds of devices are often thought of as simply very small wires, the purpose and operation of stylets is significantly different as compared to guidewires.
Conventional intravascular procedures typically involve an initial step of introducing and routing a guidewire through a patient""s vascular system to provide a rail or track along which additional intravascular devices may be introduced. Once a guidewire is in place, a sheath is routed over at least a portion of the guidewire to provide a larger opening into the vein or artery and sometimes to protect the inside walls of the vessels along the route of the guidewire. With the sheath in place, the guidewire may be removed or may remain in place as additional intravascular devices, such as intravascular leads and catheters, are introduced into the patient""s vascular system.
To better accomplish the purpose of a guidewire providing a track along the patient""s vascular system for other intravascular devices, it is desirable that the guidewire be extremely flexible and preferably have the ability to vary the flexibility of the distal tip and/or deflect the distal tip so as to aid in routing the guidewire through the patient""s vascular system. U.S. Pat. Nos. 4,215,703, 4,456,017, 4,719,924, 4,757,827, 4,886,067 and 5,060,660 describe designs for guidewires that use an internal tensioning member or core wire to alter the characteristics of the non-expandable distal tip and/or to deflect the distal tip. U.S. Pat. Nos. 4,271,845 4,822,345, 5,605,162, 5,762,615, 5,851,203, 5,957,903 and 6,183,420 describe various designs for guidewires with adjustable stiffness by moving a core member axially within the guidewire. U.S. Pat. Nos. 5,938,623 and 6,039,743 describe a guidewire with adjustable stiffness that is controlled by running electricity through a memory metal wire tip. Flexibility of the guidewire has also been provided by gradually tapering some portion of the distal end as described in U.S. Pat. Nos. 5,851,203 and 5,916,178 or by changing materials at the end of the guidewire as described in U.S. Pat. Nos. 6,017,319 and 6,068,623. The flexibility of guidewires has also been enhanced by making cuts or slots in the distal region of the guidewire as shown in U.S. Pat. Nos. 3,802,440, 5,437,288, 5,605,543, 5,833,632, 6,004,279 and 6,017,319. Similar arrangements for increasing the flexibility of the catheter that is tracked over the guidewire are also described in U.S. Pat. Nos. 5,304,131, 5,315,996, 5,322,064, 5,441,483, 5,573,520, 5,743,876 and 6,048,339. Catheter arrangements capable of producing compound bends are described in U.S. Pat. Nos. 5,758,656 and 5,820,591.
In contrast to the guidewire that serves as a track over which other intravascular devices are routed, a stylet is used within an internal lumen of an intravascular device both to push that device through the vascular system and to steer the device as it is being pushed. Although some intravascular devices are designed to steer themselves using internal core wires, almost all leads, most catheters and some guidewires have an inner channel or lumen into which a stylet is inserted. In addition to pushing the intravascular device through the vascular system by engaging the distal end of the device, the stylet also serves to deflect the distal end of the intravascular device so as to steer the distal end through the vascular system. Unlike the lead, catheter or guidewire, which is highly flexible and floppy, the stylet must be stiffer and more rigid so as to enable the stylet to push the lead or catheter through the patient""s vascular system. In addition, guidewires have diameters that are typically at least twice as large (0.030-0.040 inches) as the diameters of stylets (less than 0.016 inches) because guidewires are most often formed of a coiled wire, instead of a straight tubular wire.
Conventionally, stylets having different bends on the distal end are used at different points of advancing the lead or catheter to a desired location. For straight segments of a vessel a straight stylet is used, whereas a stylet with a curved distal tip is used to navigate the lead or catheter through a curved portion of a vessel. U.S. Pat. No. 2,118,631 shows an early stylet formed of coils of flat wire welded to plugs at both ends that could be bent by the physician into either a straight or curved configuration at its distal end prior to insertion into the lumen of a catheter or the like. U.S. Pat. Nos. 4,498,482 and 4,796,642 show early examples of solid wire stylets. While such conventional stylets can be used effectively in the hands of a skilled surgeon, the process can be complicated and time consuming. Implantation of a lead with a conventional stylet often involves multiple insertions and withdrawals of the stylet, with the surgeon adjusting the bend on the distal end so as to be able to continue to advance the stylet and lead into a desired position. One type of lead placement that is particularly complicated is the placement of a J-shaped lead in the atrial chamber. In this procedure, a straight stylet is used to advance the lead into the atrial chamber of the heart. Once there, a J-shaped stylet is used to force the lead to bend back on itself in order to be secured in a desired location in the atrium.
To overcome the problem of having to repeatedly insert and withdraw a stylet in order change the shape of the distal end, attempts have been made to develop a steerable stylet. In a steerable stylet an operator uses a handle at the proximal end of the stylet to control the direction of deflection of the distal tip of the stylet while it is in place in the lumen of the lead or catheter as it is moved along the veins or arteries. Typically, a steerable stylet is arranged as a stylet wire having a lumen within which a core wire is positioned with the distal ends of the two wires being attached. The handle is used to create a differential tension between the core wire and the stylet wire so as to deflect the distal end of the stylet as a result. Examples of such steerable stylets with deflectable tips are shown in U.S. Pat. Nos. 4,209,019, 5,396,902, 5,439,006, 5,662,119, 5,674,271, 5,824,031, 5,873,842, 6,027,462, 6,059,739, and 6,203,506. Other examples of steerable stylets can be found in PCT Publ. No. WO 00/22981 and publications describing the Placer(trademark) steerable stylet and the Locator(trademark) steerable stylet.
In U.S. Pat. No. 5,752,915, a steerable stylet uses an operating slide to retract a stylet sleeve that is not attached to the stylet wire so as to selectively expose a pre-bent distal portion of a stylet wire to deflect the distal end of an electrode lead. The handle of this stylet includes a spring for the purpose of enclosing the proximal end of the stylet wire in a tight brace to prevent buckling of the stylet wire due to friction between the stylet wire and the stylet sleeve as the stylet sleeve is retracted. In U.S. Pat. No. 5,327,906, a stress relief sleeve is provided at the junction of the stylet wire and a nose cone for the handle that allows the stylet to be removably mounted in the handle. A stop pin is also used to limit the rearward travel of a slide for the purpose of controlling the degree of curvature of the distal end of the stylet by using the stop pin to change the length of the slide. In U.S. Pat. No. 6,132,390, a conical tip is also provided to minimize stress on the stylet. A flexible plastic or polymeric jacket is disposed over the stylet wire to prevent kinking or catastrophic inelastic failure of the distal tip under a radial load.
While the advantages of a steerable stylet are apparent, none of the existing designs for steerable stylets has achieved widespread acceptance. One of the principle challenges in designing an effective steerable stylet is creating a robust design that enables a large curvature of the distal end, preferably more than 120xc2x0 so as to accommodate placement of J-shaped leads or alternatively creating sigmoidal or compound bends, while at the same time providing the flexure strength to permit repeated bending of the stylet without fracture or breaking. Unlike guidewires or catheters, these design considerations must be developed to operate under conditions where the stylet is within and constrained by the lumen of the lead or catheter that is being implanted. It would be desirable to provide a steerable stylet that could overcome these challenges and provide a robust design capable of large or compound curvatures that safely allows for repeated bending of the stylet without fracture or breaking.
The present invention provides a steerable stylet for use within a lumen of an intravascular device. The stylet includes a stylet assembly and a handle. The stylet assembly has a distal end portion and a proximal end portion and includes a tubular stylet wire having a lumen and a core wire positioned within the lumen with the distal end portion secured to the stylet wire proximate the distal end portion of the stylet wire. Unlike a conventional guidewire, the stylet wire has diameter of less than 0.016 inches and a beam strength of at least 0.005 lbf as measured by the ASTM E855-90 3-point bend test. The core wire has a distal end portion and a proximal end portion and is positioned within the lumen of the stylet wire with the distal end portion secured to the stylet wire proximate the distal end portion of the stylet wire. The handle includes a hand-held housing structure connected to one of the proximal end portion of the stylet wire or the core wire. In one embodiment, an adjustable tensioner is connected to the other of the proximal end portion of the stylet wire or the core wire to adjust a relative tension force applied between the stylet wire and the core wire. A tension limiter is arranged to limit the tension force to a limit force that is less than a breaking stress force of the stylet wire when the stylet wire is positioned within the lumen of the intravascular device. In another embodiment, a plurality of notches are defined in one or more regions of the distal portion of the tubular stylet wire to aid in the safe and effective creation of large and/or compound curves. Preferably, the core wire is secured within the tubular stylet wire without heating either wire so as to prevent any annealing of the materials that would decrease the tensile strength of the distal end portion of the stylet.
By limiting the tension force that can be applied between the core wire and the stylet wire to a force that is less than the breaking stress force of the stylet wire, the present invention prevents the stylet wire from failing as a result of an excessive stress force. This prevents an operator from overexerting the stylet wire by attempting to deflect the stylet in a situation where the intravascular device cannot be deflected, such as for example within a blood vessel. Unlike a design that provides for a fixed dimensional travel of the adjustable tensioner, the present invention provides for a floating or variable amount of travel up to a maximum deflection force. As a result, the stylet may be repeatedly deflected numerous times without causing either a stress or fatigue failure of the stylet wire. In one embodiment, the steerable stylet of the present invention is designed for implantation of cardiac J-leads and can be deflected up to a maximum deflection of at least 180xc2x0 from an original position of the stylet wire and the deflections can be repeated at least fifty times without inducing stress or fatigue failure in the stylet wire. In another embodiment, the steerable stylet of the present invention is designed for use in neurological applications and can be deflected up to a maximum of 90xc2x0 and can also be configured to create compound curves.
In a preferred embodiment, a series of at least ten notches are defined along a distal region of the stylet wire to prevent the stresses induced in the distal region of the stylet wire from kinking or bending the stylet when the distal end portion of the stylet is deflected by an operator. Preferably, the series of notches in the distal region includes at least a portion of the notches that have a progressively decreasing depth distally to proximally along the series. More preferably, the portion of the notches having a progressively decreasing depth has a constant decrease in depth between adjacent notches. In this embodiment, the distal region is preferably defined beginning between 0.050 inches and 1.000 inches proximal to the distal end of the stylet wire. Preferably, there are at least twenty-five notches of between 0.005 inches and 0.015 inches longitudinal width with a spacing between adjacent notches of between 0.010 inches and 0.050 inches and a depth of at least ten of the most distal notches of said series being approximately equal to a radius of the stylet wire minus a wall thickness of the stylet wire. In an alternate embodiment, more than one series of notches are defined in the distal region of the stylet wire to create compound curves, including curves oriented in two different planes. Preferably, the spacing and dimensions of the notches are different between the different series so as to stage the sequence in which each region associated with a given series of notches will induce a curve in response to tension on the core wire.
In a preferred embodiment, the proximal end portion of the core wire is fixedly connected to the adjustable tensioner and the tension limiter has a first end portion fixedly connected to the housing structure of the handle and a second end portion operably connected to the adjustable tensioner. Preferably, the breaking stress force of the stylet wire is at least six pounds and the limit force of the tension limiter is less than four pounds. In one embodiment, the tension limiter is a constant force spring. In another embodiment, the tension limiter is an elastomer member with a maximum compressive retention force less than the breaking stress force of the stylet wire.
Preferably, the core wire is secured within the tubular stylet wire without heating either wire so as to prevent any annealing of the materials that would decrease the tensile strength of the distal end portion of the stylet. In one embodiment, the core wire is taper ground to provide a smaller diameter core shaft while leaving a bulbous distal end having a diameter substantially equal to an outer diameter of the tubular stylet wire. The core wire is slide into the lumen of the tubular stylet wire and the bulbous distal end of the core wire is secured in place using adhesives together with a chamfered fit. By avoiding the use of heat for welding or thermal expansive fits, the core wire and stylet wire are not annealed and the corresponding decrease in tensile strength of the wires (to about 60% of their original tensile strength) is not encountered.
In one embodiment, the handle includes a mechanism for providing tactile psuedo-feedback to an operator that is generally indicative of the relative tension force without directly engaging the tension limiter. In another embodiment, the beam strength of the stylet wire is preferably about 0.010 lbf so as to be sufficient to cause the stylet wire to return to at least an original position as the relative tension force is removed from the stylet wire and preferably to an angle beyond its original position.